Carnot cycle systems are commonly used as heat transfer machines allowing one room or volume to be cooled as another is heated. There have been continuous attempts to improve the efficiency of Carnot cycle refrigeration machines. Many refrigeration devices utilize a hermetically sealed compressor that requires special cooling so as to not overheat during operation.
Vapor-compression refrigeration has been widely used as a method for air-conditioning large public buildings, private residences, hotels, hospitals, theaters, restaurants, and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor compression refrigeration systems.
All such systems have four basic components: a compressor, a condenser, an expansion valve, and an evaporator. To begin the refrigeration cycle within a vapor compression refrigeration system, circulating refrigerant enters the compressor in a thermodynamic state known as a “saturated vapor.” A saturated vapor is a vapor at its saturation temperature and pressure. In other words, a saturated vapor is a vapor whose temperature and pressure are such that any compression of its volume at constant temperature causes it to condense to liquid at a rate sufficient to maintain a constant pressure. The saturated vapor is compressed in a compressor to a higher pressure, resulting in an increase in temperature of the refrigerant. The hot, compressed refrigerant enters the thermodynamic state known as a “superheated vapor.” A superheated vapor is a vapor that is at a temperature higher than the saturation temperature corresponding to its pressure. In other words, the superheated vapor is at a temperature and pressure at which it can be condensed with, for example, ambient air or a cooling fluid such as water. In most systems, the hot, compressed vapor is routed through a condenser where it is cooled and condensed into a liquid as it flows through a coil or tubes with cool water or cool air flowing across the coil or tubes. It is within the coil or tubes where the circulating refrigerant rejects heat from the system (i.e. away from the space to be cooled).
The condensed liquid refrigerant, now in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. A saturated liquid is a liquid at its saturation temperature and saturation pressure. In other words, a saturated liquid is a liquid whose temperature and pressure are such that any decrease in pressure without change in temperature causes it to boil. The pressure reduction caused by the expansion valve results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be cooled.
The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space. The evaporator is where the circulating refrigerant absorbs and removes heat, which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.
Finally, the refrigeration cycle is completed as the refrigerant from the evaporator is again routed back into the compressor. The cycle begins again as the circulating refrigerant enters the compressor.
Various devices and methods have been developed to cool hermetically sealed compressors. Many previous attempts utilize either a portion of the returning cool refrigerant or an additional heat exchanger specifically designed to cool the compressor. Alternative methods include cooling the hot exhaust gas exiting the compressor, cooling the condenser coil assembly, and utilizing the returning cool refrigerant to cool the warm liquid refrigerant as it exits the condenser coil assembly.
An alternative attempt to cool the compressor including cooling the compressor's lubricating oil. This has been done by diverting cold evaporative gases through a cooling loop built into the bottom of the compressor. However, while this extends the life of the compressor, warmer evaporated gases are then fed back into the refrigeration cycle for compression. However, methods utilizing the returning cold evaporated gases to cool portions of the system, including the compressor, add heat to the overall system and result in less efficient cooling.
Similar attempts to cool the compressor include pumping the compressor's lubricating oil through a system of tubes to an external heat exchanger where ambient air cools the oil before it returns to the compressor. Though this does not add heat to the cold evaporative gases, the relative cooling efficiency is minimal and very dependent on the fin area of the external heat exchanger. As the temperature of the ambient air within such casings often reaches temperatures between 120° F. to 140° F., the amount of cooling achieved by such heat exchangers is minimal at best.
In another more recent attempt, a portion of the cold evaporative gases is diverted to cool the casing of the hermetically sealed compressor. However, this also adds heat to the cold evaporative gases, significantly reducing the overall efficiency of the whole refrigeration process. In fact using the cold evaporative gases to cool other portions of the system can reduce overall efficiency by as much as 20%.
The prior art allows a significant decrease in efficiency in order to prolong the life of the compressor by lowering its operating temperature. As previously mentioned many of these systems utilize the returning cold evaporative gases to cool either the compressor itself or to cool the hot gas emitted from the compressor before they enter the condenser.
Many alternative systems fail to reach maximum efficiency because they attempt to gain more cooling than is available through auxiliary and additional heat exchangers. For example, condensate recovered from the evaporative portion of the Carnot system is used to remove heat from the hot gases. However, many prior art systems attempt to use the condensate to cool more than just the hot gas emitted from the compressor. Prior art systems attempt to use the condensate to cool the gases at many locations in the system, such as before and after the condenser. While this is done in an attempt to exploit all the heat-absorbing capabilities of the recovered condensate, the result is that the condensate becomes too hot. Consequently, the heat removed before the condenser is reintroduced back into the system after the condenser.
In sum, large quantities of condensate formed on the evaporator must be either drained or evaporated by hot sections of the Carnot cycle. Prior art devices utilizing the returning cold evaporative gases to cool the compressor are extremely inefficient. Other methods over-utilize recovered condensate and thereby reintroduce heat back into the system. Finally, prior art systems attempting to address these issues are complex and require replacement or significant modification of existing refrigeration machines. The present system and method provides several novel methods of cooling the compressor and refrigerant of a Carnot system, including methods that efficiently use condensate recovered from the evaporator.